Superoxide dismutase activity was shown to be present in bovine milk serum and was quantified by measuring the capacity of retentate from dialyzed milk serum to inhibit reduction of cytochrome c by xanthine-xanthine oxidase-generated superoxide anion. One unit of enzyme was defined as the quantity of superoxide dismutase which inhibits cytochrome c reduction by 20%. By this definition 19,500 units of enzyme were present per liter of retentate from dialyzed milk serum. This amount is equivalent to about 2.4 mg of purified bovine erythrocyte superoxide dismutase per liter. Polyacrylamide gel electrophoresis of a partially-purified superoxide dismutase from acid whey, followed by staining for enzymic activity, confirmed the presence of the enzyme in milk serum which was identical in electrophoretic properties to those of bovine erythrocyte copper-zinc superoxide dismutase. Pasteurization at 63 C for 30 min did not decrease superoxide dismutase activity in milk serum. Heating of purified bovine erythrocyte-superoxide dismutase at 100 C for 1 min resulted in almost complete loss of enzymic activity, whereas the partially-purified superoxide dismutase from acid whey still retained 40% of the original activity under these conditions. Bovine milk superoxide dismutase may be an important naturally-occurring antioxidant for increasing oxidative stability of milk and other dairy products.
1University of Wisconsin.
2University of Kentucky.